1. Field of the Invention
The present invention relates to an optical fiber amplifier for controlling gain flatness, and more particularly, to an optical fiber amplifier for controlling gain by measuring the gain flatness thereof.
2. Description of the Related Art
In an optical transmission system, an erbium doped fiber amplifier (EDFA) directly amplifies an optical signal that has been weakened during transmission without converting the optical signal into an electrical signal, thus remarkably extending the transmission distance. U.S. Pat. No. 5,768,012 to Raymond Zanoni et al. entitled Apparatus And Method For The High-Power Pumping Of Fiber Optic Amplifiers and U.S. Pat. No. 5,673,129 to Victor Mizrahi entitled WDM Optical Communication Systems With Wavelength Stabilized Optical Selectors are examples of known optical amplifiers.
Conventional optical fiber amplifiers are optimized to give the highest level of gain flatness before being installed on a transmission path. In an optical fiber amplifier, automatic gain control or automatic level control is achieved by monitoring the power levels of all input signals and the power levels of all output signals.
Due to a change in the signal power level for each channel that is added or dropped during add/drop multiplexing (ADM) as well as a change in loss at each different wavelengths which depends on the properties of an optical fiber used as a transmission path, there exhibits a great difference in the quality of signals of the respective channels received by a reception terminal.
In general, an optical fiber used as a transmission path exhibits a great difference in loss per channel. If a light signal in the wavelength band of 1545.32 to 1557.36 nm, which is generally used in a transmission system having 16 channels at intervals of 100 GHz, is transmitted over e.g., 100 Km, the loss experienced by a light signal in the 1545.32 nm channel is 0.3 dB greater than that experienced by a light signal in the 1557.36 nm channel.
Also, in a network system in which ADM is used, since the output power levels of a typical laser diode are not uniform, it is quite difficult to attain coincidence of power levels between a channel signal transmitted through an ADM node and a channel signal add-multiplexed after being drop-multiplexed.
Thus, since the power level and the signal-to-noise ratio vary greatly between channels of the light signal received by the reception terminal, the signal quality of a channel compared to that as designed may be considerably degraded. If this phenomenon repeatedly occurs, the problem becomes severe.
To solve the above problems, it is an objective of the present invention to provide an optical fiber amplifier for use in a long-distance transmission system or an add/drop multiplexing (ADM) system, for controlling gain flatness by measuring the gain flatness of an output signal of the optical fiber amplifier and controlling the amplification gain thereof.
Accordingly, to achieve the above objective, there is provided an optical fiber amplifier for controlling gain flatness including an optical fiber amplifying unit for amplifying input signal light according to a predetermined amplification control value, a measuring unit for measuring the power level of amplified spontaneous emission (ASE) at a predetermined wavelength band output from the optical fiber amplifying unit, and a gain controller for controlling the amplification control value of the optical fiber amplifying unit according to the measured power level of the ASE to flatten the gain of the optical fiber amplifying unit.